1. Field of the Invention
The present invention relates to a spring-return switch having a movable contact held on a slider integrally provided with an operating rod, having a first contact arm in continuous contact with a common contact and a second contact arm to be brought into contact with a transfer contact for switching, and to a dual spring-return switch provided with a pair of switch units respectively having sliders integrally provided with operating rods, to be operated selectively for switching and disposed at a specified distance from each other with their operating rods extending in parallel to each other.
2. Description of the Related Art
Referring to FIG. 7 showing a known spring-return switch, a case 1 formed of a synthetic resin is provided integrally with a common contact 2 and a transfer contact 3 embedded respectively in the respective inner surfaces of opposite side walls 1a and 1b by insert molding, and the common contact 2 and the transfer contact 3 are connected respectively to terminals, not shown, provided externally on the case 1. Contained in the case 1 are a slider 5 formed of a synthetic resin by molding and integrally having a push rod 4 extending through a through hole 1c formed in the central portion of the upper wall of the case 1 so as to be axially movable, a movable contact 6 formed by bending a elastic, thin metal plate in a shape substantially resembling the letter U and fixed to the lower end of the slider 5, and a return spring 7 extended between the slider 5 and the lower wall of the case 1 to bias the slider upward. The movable contact 6 has a flat base portion 6a, a first contact arm 6b and a second contact arm 6c extending downward respectively from the opposite ends of the base portion 6a. The first contact arm 6b is continuously in contact with the common contact 2. Normally, the second contact arm 6c is in contact with the inner surface of the side wall 1b, as shown in FIG. 7. When the slider 5 is lowered, the second contact arm 6c comes into contact with the transfer contact 3.
In FIG. 7, the spring-return switch is in the OFF-state in which the common contact 2 is disconnected from the transfer contact 3. When the push rod 4 projecting upward from the upper surface of the case 1 is depressed by a predetermined distance by an actuator, not shown, to set the spring-return switch in the ON-state, the second contact arm 6c comes into contact with the transfer contact 3 to connect the common contact 2 to the transfer contact 3. When the pressure applied to the push rod 4 is removed while the spring-return switch is in the ON state, the compressed return spring 7 pushes up the slider 5 and, consequently, the second contact arm 6c is separated from the transfer contact 3 to disconnect the common contact 2 automatically from the transfer contact 3.
FIG. 8 shows a spring-return switch for use on automobiles or the like, in which parts like or corresponding to those described previously with reference to FIG. 7 are denoted by the same reference characters. This spring-return switch is similar in construction and function as the spring-return switch shown in FIG. 7. The spring-return switch shown in FIG. 8 is designed to obviate undesirable, vibratory sliding movement of the movable contact attributable to the vibration of the slider. As shown in FIG. 8, the spring-return switch is provided with a slider 5 provided with a slot, and a movable contact 6 having a base portion 6a extending through the slot of the slider 5 with a clearance C between its upper surface and the upper surface of the slot to allow idle movement of the slider 5. If the clearance C is not secured between the slider 5 and the base portion 6a of the movable contact 6, the movable contact 6 will vibrate when the slider 5 vibrates to promote the abrasion of the first contact arm 6b and the second contact arm 6c of the movable contact 6, and a common contact 2, which will deteriorate the performance of the spring-return switch.
Referring to FIGS. 10 and 11 showing a conventional dual spring-return switch having two switch units, a case 11 having a shape substantially resembling a rectangular cuboid is constructed by combining a lower case 12 having opposite longer side walls 12a and 12b, and an upper case 13 having bosses. A common contact 14 is embedded in the inner surface of the longer side wall 12a and a pair of transfer contacts 15 and 16 are embedded in the inner surface of the other longer side wall 12b with a space therebetween. Terminals 14a, 15a and 16a projecting outward from the outer surface of the longer side wall 12a are connected respectively to the extension of the common contact 14 buried in the longer side wall 12a and the extensions of the transfer contacts 15 and 16 buried in the opposite shorter side walls 12c and 12d. The common contact 14, the transfer contacts 15 and 16, and the terminals 14a, 15a and 16a are combined with the lower case 12 by insert molding. Sliders 17 and 18 integrally provided respectively with operating rods 21 and 22 are supported within the case 11 with the operating rods 21 and 22 extending in parallel to each other respectively through parallel holes 13a formed in the bosses of the upper case 13. The upper ends of the operating rods 21 and 22 are inserted respectively in a pair of holes formed in an external device, not shown. In most cases, the interval between the operating rods 21 and 22 is determined on the basis of the center distance between the holes of the standardized external device. A movable contact 19 fixed to the slider 17 has a contact arm continuously in contact with the common contact 14, and another contact arm to be separated from the transfer contact 15 when the operating rod 21 is depressed. A movable contact 20 fixed to the slider 18 has a contact arm continuously in contact with the common contact 14, and another contact arm to be separated from the transfer contact 16 when the operating rod 22 is depressed. The sliders 17 and 18 are biased upward by a pair of return springs 23 (only one of them is shown) extended between the lower surfaces of the sliders 17 and 18 and the lower wall of the case 11.
When the dual spring-return switch is in the OFF-state, the sliders 17 and 18 are pressed against the upper wall of the case 11 by the return springs 23, so that the common contact 14 is connected to the transfer contact 15 by the movable contact 19, and the common contact 14 is connected to the transfer contact 16 by the movable contact 20. When the operating rod 21 (22) is depressed against the resilience of the return spring 23, to lower the slider 17 (18) by a predetermined distance, the movable contact 19 (20) is separated from the transfer contact 15 (16) to disconnect the common contact 14 from the transfer contact 15 (16). The depression of the operating rod 21 can be detected through the detection of disconnection of the terminals 14a and 15a, the depression of the operating rod 22 can be detected through the detection of disconnection of the terminals 14a and 16a.
The spring-return switch shown in FIG. 7 is designed so that the second contact arm 6c comes into contact with the transfer contact 3 when the operating rod 4 is depressed axially by a predetermined distance. However, it occurs sometimes that the push rod 4 is depressed diagonally. Particularly, when the operating rod 4 is depressed in an inclined position as shown in FIG. 9 that increases the resistance of the inner surface of the side wall 1b against the sliding movement of the second contact arm 6c, the first contact arm 6b slides downward before the second contact arm 6c slides downward. Therefore, the operating rod 4 needs to be depressed by a distance exceeding the predetermined distance to bring the second contact arm 6c into contact with the transfer contact 3, which delays the switching operation of the spring-return switch greatly. Since the effective stroke of the operating rod 4 is dependent on the inclination of the operating rod 4, the switching performance of the spring-return switch is unreliable.
The spring-return switch shown in FIG. 8 has the clearance C between the slider 5 and the base portion of the movable contact 6 to intercept the transmission of the vibration of the slider 5 to the movable contact 6. Therefore, the movable contact 6 tends to tilt in one direction even if the operating rod 4 is depressed axially and hence it is difficult to time the switching operation of the spring-return switch correctly.
The dual spring-return switch shown in FIGS. 10 and 11 has the terminals 14a, 15a and 16a projecting from the outer surface of one of the longer side walls of the case 11. However, a desire to use a dual spring-return switch having three terminals projecting from the outer surface of one of the shorter side walls of the case has grown recently with the recent progressive increase in the packaging density of printed wiring boards with which the dual spring-return switch is to be used, because of many restrictions on the layout of components of printed wiring boards.
Dual spring-return switches previously proposed to meet such a requirement are shown in FIGS. 12 and 13, in which like or corresponding parts are denoted by the same reference characters.
The dual spring-return switch shown in FIG. 12 has a lower case 12 having opposite longer side walls 12a and 12b, and opposite shorter side walls 12c and 12d, a common contact 14 embedded in the inner surface of the longer side wall 12a, transfer contacts 15 and 16 separately embedded in the inner surface of the other longer side wall 12b, and terminals 14a, 15a and 16a buried in and projecting from the shorter side wall 12c. The extensions of the transfer contacts 15 and 16 are extended through the longer side wall 12b and connected respectively to the terminals 15a and 16a, and the extension of the common terminal 14 is extended through the other longer side wall 12a and connected to the terminal 14a. This construction needs the longer side wall 12b in a comparatively large thickness to secure satisfactory reliability of the dual spring-return switch, which, inevitably increases the size of the dual spring-return switch.
The dual spring-return switch shown in FIG. 13 has a substantially U-shaped common contact 14 having a first contact arm 14b to be in contact with a movable contact 19, and a second contact arm 14c to be in contact with a movable contact 20. The common contact 14 is disposed between the movable contacts 19 and 20 in a lower case 12 having opposite longer side walls 12a and 12b and opposite shorter side walls 12c and 12d. Transfer contacts 15 and 16 are embedded respectively in the inner surfaces of the opposite shorter side walls 12c and 12d. This construction requires a complex bending process of forming the common contact 14 in predetermined dimensions so that the common contact 14 is able to be in contact properly with the movable contacts 19 and 20 when a pair of operating rods 21 and 22 are arranged at a predetermined interval, which inevitably increases the cost of the dual spring-return switch.